Reverse-circulation, center-discharge drilling (RCCD) through concentric tubing is a proven method for minimizing formation damage while drilling producing formations, such as tight gas sand and coal bed methane. Because RCCD drilling returns cuttings through the inner diameter of a double-wall drill pipe, it does not expose the formation to possible damage from drilling fluid and cuttings.
This technique is accomplished with a concentric rotary drill string and a center discharge drill bit. A vacuum may be applied at the surface to reduce the bit face pressure to a level below the formation pore pressure, to further reduce the potential for formation damage; however, the vacuum assist from this approach is limited.
The deployment of concentric jointed tubing represents significant additional time and cost for drilling the well to completion. Concentric coiled tubing (CCT) can speed the deployment time, and allows continuous drilling operations in the producing formation. Drilling operations using coiled tubing requires a motor to turn the drill bit. Rotary drilling motors capable of operating on dry gas with a center discharge are not available.
It is generally desirable to operate a drill motor on dry gas for completion drilling of water sensitive formations. Progressive cavity motors incorporate elastomeric stators that degrade rapidly when operated on dry gas. Turbodrills are capable of operation on gas, but these tools stall easily when operated on gas, and the motor speed is generally much too high for effective drilling. These motors also tend to be very long, which limits steerability. A previous attempt to develop a gas turbine motor for drilling application involved the use of a multi-stage planetary gear, to increase torque and reduce the speed, to drive a conventional roller cone drill bit. The relatively high cost and complexity of the multistage planetary gearbox prevented commercial acceptance of that design. Further, the transmission employed in that design was not suited for a center discharge passage.
It would be desirable to provide a compact, steerable gas turbine motor and a speed reduction transmission suitable for RCCD drilling, capable of providing the speed and torque required for drilling with conventional roller cone or polycrystalline diamond compact (PDC) bits.